The present invention relates to an angular position detector having magnetoresistors.
An angular position detector having a magnetoresistor system is described in U.S. Pat. No. 4,629,982. A magnetoresistor comprises a permanent magnet and two electrical resistors varying as a function of magnetic flux and arranged in a compact assembly. The magnetoresistor is attached to a fixed part, or stator, in front of which a ferromagnetic disc or rotor turns. The rotor has a serrated structure defining a periodic pattern on it. The magnetic flux passing through each resistor varies as a function of the distance of the teeth closest to this resistance. The bridge of resistance thus becomes unbalanced, which is measured by an appropriate electrical circuit. The signal obtained, which is more or less sinusoidal in shape and the period of which corresponds to the angular pitch of the spacing between the teeth, varies according to a law.
Magnetoresistors offer numerous advantages; they are compact, low cost, require no maintenance and can function at temperatures varying greatly from ambient temperature, particularly at very low temperatures, without being affected. However, they do have some disadvantages. One of the most noteworthy disadvantages is that they produce torque on the rotor and the shaft supporting it so that measurements of angular position are adversely affected and it is not possible to obtain very precise results, particularly if the part supporting the rotor is activated by a motor with low residual torque. Moreover, since the polar space between the rotor and the stator is slight, manufacturing inaccuracies have important consequences for measurement. Finally, in a certain number of cases, it would be preferable to make use of not only what is known as the electrical angle between the rotor and the stator (bearing in mind that a 360.degree. electrical angle corresponds to one step of the rotor structure), but also of the mechanical angle which these parts form.